Hyperthyroidism - Diagnosing overactive thyroid

See your GP if you think you may have an overactive thyroid gland (hyperthyroidism).

A diagnosis will be based on your symptoms and the results of blood tests that assess how well your thyroid gland is working. These are known as thyroid function tests.

Thyroid function tests

Your GP will take a sample of your blood and test it for levels of:

• thyroid-stimulating hormone (TSH)
• thyroxine and triiodothyronine (the thyroid hormones)

TSH is made in the pituitary gland in your brain and controls the production of thyroxine and triiodothyronine.

• When the level of thyroxine and triiodothyronine in your blood are normal, your pituitary gland releases a normal level of TSH. When thyroid hormone production becomes excessive, the pituitary gland stops releasing TSH.  
• When the level of thyroxine or triiodothyronine drops, the pituitary gland produces more TSH to boost it.

If you have an overactive thyroid, the thyroid function test will show that levels of TSH in your blood are consistently lower than normal. Low levels of TSH mean your thyroid gland is overactive and likely to be making excessive thyroid hormones. This is the first part of the thyroid function test.

Your GP will then test your blood for levels of thyroxine and triiodothyronine. If you have an overactive thyroid, you will have higher than normal levels of both these hormones.

Subclinical overactive thyroid gland

In some cases, tests may show you have normal thyroid hormone levels, but low or suppressed levels of TSH.

This is known a subclinical overactive thyroid gland. If you are diagnosed with subclinical overactive thyroid, you may not need treatment.

In most cases, the reduced level of TSH in your blood returns to normal within a couple of months and your subclinical hyperthyroidism will resolve by itself.

However, you will need a further thyroid function test so your condition can be monitored.

Determining the underlying cause

If tests confirm an overactive thyroid gland, you may be referred for further tests to determine the underlying cause.

An additional test that may be used is an isotope thyroid scan. This involves swallowing small amounts of a radioactive substance (an isotope), usually technetium, in capsule or liquid form.

A scan is then used to measure how much of the isotope has been absorbed by your thyroid gland.

If your thyroid gland absorbs a high amount of the isotope, it is likely the underlying cause is either Graves' disease or thyroid nodules.

If the amount is low, the underlying cause could be due to:

• swelling (inflammation) of the thyroid gland (thyroiditis), often caused by your immune system mistakenly attacking thyroid tissue or, less commonly, by infection
• having too much iodine in your diet
• in rare cases, thyroid cancer